[tip: sched/urgent] sched/fair: Fix unfairness caused by stalled tg_load_avg_contrib when the last task migrates out

["tip-bot2 for xupengbo" <tip-bot2@linutronix.de>]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     ca125231dd29fc0678dd3622e9cdea80a51dffe4
Gitweb:        https://git.kernel.org/tip/ca125231dd29fc0678dd3622e9cdea80a51dffe4
Author:        xupengbo <xupengbo@oppo.com>
AuthorDate:    Wed, 27 Aug 2025 10:22:07 +08:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Sat, 06 Dec 2025 10:03:13 +01:00

sched/fair: Fix unfairness caused by stalled tg_load_avg_contrib when the last task migrates out

When a task is migrated out, there is a probability that the tg->load_avg
value will become abnormal. The reason is as follows:

1. Due to the 1ms update period limitation in update_tg_load_avg(), there
   is a possibility that the reduced load_avg is not updated to tg->load_avg
   when a task migrates out.

2. Even though __update_blocked_fair() traverses the leaf_cfs_rq_list and
   calls update_tg_load_avg() for cfs_rqs that are not fully decayed, the key
   function cfs_rq_is_decayed() does not check whether
   cfs->tg_load_avg_contrib is null. Consequently, in some cases,
   __update_blocked_fair() removes cfs_rqs whose avg.load_avg has not been
   updated to tg->load_avg.

Add a check of cfs_rq->tg_load_avg_contrib in cfs_rq_is_decayed(),
which fixes the case (2.) mentioned above.

Fixes: 1528c661c24b ("sched/fair: Ratelimit update to tg->load_avg")
Signed-off-by: xupengbo <xupengbo@oppo.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Aaron Lu <ziqianlu@bytedance.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Tested-by: Aaron Lu <ziqianlu@bytedance.com>
Link: https://patch.msgid.link/20250827022208.14487-1-xupengbo@oppo.com
---
 kernel/sched/fair.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 769d7b7..da46c31 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4034,6 +4034,9 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
 	if (child_cfs_rq_on_list(cfs_rq))
 		return false;
 
+	if (cfs_rq->tg_load_avg_contrib)
+		return false;
+
 	return true;
 }